∑j12⁢j34(2⁢j12+1)⁢(2⁢j34+1)⁢(2⁢j13+1)⁢(2⁢j24+1)⁢{j1j2j12j3j4j34j13j24j}⁢{j1j2j12j3j4j34j13′j24′j}=δj13,j13′⁢δj24,j24′.